1. Field of the Invention
This invention relates to heating, ventilation, and air conditioning (HVAC) systems, and more particularly to a method and apparatus for measuring airflows in such systems.
2. Background of the Invention
Because of stiffer ventilation requirements and the never ending pursuit of the most efficient HVAC system, modern HVAC units have been called upon to provide a more accurate overall temperature and ventilation control. The key to such improved HVAC system control lies in part in the degree of accuracy obtained in measuring various system parameters. One important measured parameter is the ventilation rate provided by the HVAC system to a receiving zone. The ventilation rate is a measure of the amount of outdoor air delivered to a receiving zone over a given time and is indicative of the air quality within such a zone.
Because of the relationship between indoor air quality and the amount of outdoor air received in a receiving zone, air quality standards have been promulgated requiring a minimum volume of outdoor air that must be delivered to a receiving zone at any given airflow. Standard 62-1989, set forth by ASHRAE (the American Society of Heating, Refrigerating, and Air Conditioning Engineers), is such a standard and has been adopted by many building professionals. Thus, HVAC systems in compliance with such air quality standards must include a method of accurately determining whether the required amount of outdoor air is delivered by the HVAC unit.
Other parameters essential to the proper operation and control of an HVAC system include the amount of return air received by the HVAC unit and the total airflow across the HVAC system. Only with accurate airflow measurements of these parameters can precise control of the type and amount of air delivered to a receiving zone be obtained.
A problem encountered in accurately measuring airflow in HVAC units results from an inability to accurately sense the full range of airflows utilized by a given HVAC system. For example, pressure transducers in conventional systems cannot be relied upon to provide accurate pressure measurements over the complete airflow range utilized by an HVAC unit.
Various attempts have been made to overcome this airflow measuring problem. One such approach includes amplifying the airflow velocity at the measuring location and thereby increasing the velocity pressures to a higher, more accurately measurable value. Increasing the airflow velocity can be achieved by providing a flow restriction adjacent and upstream of the measuring location and utilizing the driving fan of the HVAC unit downstream of the measuring location. This technique is inefficient because of the large amount of energy required to increase the airflow velocity at the measuring location. Further, this method only raises the measured velocity pressure corresponding to a low airflow condition of the HVAC system, it does not affect the need for a pressure transducer capable of accurately measuring velocity pressures over the total airflow range of the HVAC system.
For the foregoing reasons and others, a need exists for a method and apparatus that accurately measures a full range of airflows across an HVAC system.